1. Field of the Invention
This invention relates to a diaphragm type fuel pump which feeds fuel in response to reciprocations of a diaphragm.
2. Description of the Related Art
A diaphragm type fuel pump has been utilized as a fuel supply for feeding fuel to a fuel injector from a fuel tank. An example of such a fuel pump is shown in FIG. 8 of the accompanying drawings.
A diaphragm type fuel pump 10 comprises: a first body 16 including a fuel introducing path 12 and a fuel discharging path 14; a second body 18 arranged on one side of the first body 16; a cover 20 arranged on the other side of the first body 16; a diaphragm 22 sandwiched between the first and second bodies 16 and 18; and a membrane 24 sandwiched between the first body 16 and the cover 20.
A pump chamber 28 is formed between the diaphragm 22 and the first body 16, while a pulse chamber 30 is formed between the diaphragm 22 and the second body 18. The pump chamber 28 communicates with both the fuel introducing path 12 and fuel discharging path 14 of the first body 16. The second body 18 is provided with a pulse introducing path 32 in order to introduce pulse pressure to the pulse chamber 30. The pulse pressure is generated by an engine and is supplied to the pulse chamber 32 via the pulse introducing path 32.
A fuel sucking chamber 34 communicating with a fuel tank (not shown) and a fuel discharging chamber 35 communicating with a fuel injector (not shown) are formed between the membrane 24 and the first body 16. Between the membrane 24 and the cover 20, a damping chamber 36 faces the fuel sucking chamber 34 via the membrane 24, and a damping chamber 37 faces the fuel discharging chamber 35 via the membrane 24.
The fuel sucking chamber 34 communicates with the pump chamber 28 via the fuel introducing path 12 of the first body 16, while the fuel discharging chamber 35 communicates with the pump chamber 28 via the fuel discharging path 14 of the first body 16. A check valve 38 is provided in the fuel introducing path 12 in order to feed fuel only to the pump chamber 28 from the fuel sucking chamber 38. Further, a check valve 40 is provided in the fuel discharging path 14 in order to feed fuel only to the fuel discharging chamber 35.
In this diaphragm type fuel pump 10, pulse pressure generated in a crank chamber (not shown) of the engine is introduced into the pulse chamber 30, thereby reciprocating the diaphragm 22 between the pump chamber 28 and the pulse chamber 30. As a result, fuel introduced into the fuel sucking chamber 34 from the fuel tank is supplied to the fuel injector via the pump chamber 28 and the fuel discharging chamber 35.
The diaphragm 22 is generally made of a rubber or synthetic resin material. The rubber material becomes hard at a low temperature, and tends not to reciprocate smoothly, thereby reducing the flow rate of the fuel pump. On the contrary, a synthetic resin material that remains flexible regardless of temperature variations has been utilized for snow mobiles or the like which are structured so as to be usable in very cold areas.
FIG. 9 shows the diaphragm 22 made of only synthetic resin in the related art. The diaphragm 22 is flat, and has openings 42 at four corners in which screws (not shown) are received in order to fixedly hold the first body 16 and two lids 18 and 20.
At normal temperatures, the synthetic resin is hard compared with the rubber material, so that the synthetic resin diaphragm 22 is less flexible than the rubber diaphragm, and takes time to reciprocate. The fuel pump including a synthetic resin diaphragm 22 therefore suffers from a reduced flow rate compared with a fuel pump including a rubber diaphragm 22.
It is well-known that the flow rate of the diaphragm type fuel pump depends upon a size of an effective diameter X (shown in FIG. 8) of the diaphragm 22. The term xe2x80x9ceffective diameterxe2x80x9d means a diameter of the diaphragm in which the pumping operation is performed. Referring to FIG. 8, the effective diameter X of the diaphragm 22 is equal to a diameter of an inner wall of the second body 18 constituting the cylindrical pulse chamber 30.
FIG. 10 is a graph showing the relationship (N-Q characteristics) between the number N of pulses and flow rate Q of pumps 10 having the synthetic resin diaphragm 22 and two different effective diameters X. In FIG. 10, black squares ▪ denote the N-Q characteristics of a fuel pump having a relatively small effective diameter diaphragm (for a maximum flow rate of 42 L/H), and black circles xe2x97xaf denote the N-Q characteristics of a fuel pump having a relatively large effective diameter diaphragm (for a maximum flow rate of 72 L/H). Referring to the N-Q characteristics, it is understood that the effective diameter extensively affects the flow rate of the fuel pump.
In the related diaphragm type fuel pump 10, a variety of second bodies 18 have been prepared in accordance with required flow rates of the fuel pump. Since the different flow rates mean the necessity of different effective diameters X, the second bodies 18 have been selected in accordance with the required flow rates. As a result, a plurality of dies have been required, which has caused an increase in manufacturing costs of fuel pumps.
The invention is intended to overcome the foregoing problems of the related art, and to provide a diaphragm type fuel pump that includes a single kind of body, meets requirements for a plurality of flow rates and can be manufactured at a reduced cost.
According to the present invention, at very low temperatures, the diaphragm of the fuel pump can assure strokes identical to those of the synthetic resin diaphragm of the related art and having an effective diameter X that is the same as that of the present invention. At normal temperatures, the diaphragm of the invention can assure large strokes compared with those of the synthetic resin diaphragm, and increases necessary flow rates. Therefore, the flow rates can be varied as desired only by exchanging the diaphragm but without replacing the second body. As a result, it is not necessary to prepare a plurality of dies, which is effective in promoting the use of just one type of second body and reducing manufacturing costs.
In order to accomplish the foregoing object of the invention, there is provided a diaphragm type fuel pump comprising: a fuel sucking chamber and a fuel discharging chamber; a first body having a fuel introducing path communicating with the fuel sucking chamber and a fuel discharging path communicating with the fuel discharging chamber; a diaphragm fixed to the first body using a second body; and a pump chamber constituted by the diaphragm and the first body and communicating with the fuel introducing path and the fuel discharging path. The diaphragm includes an outer diaphragm made of resin and having an opening formed within an effective diameter of the diaphragm, and an inner diaphragm arranged in the opening of the outer diaphragm. Further, the outer and inner diaphragms are mutually fixed using an elastic coupling member.